[release-5.7] fluentd multiline exception plugin fails to detect JS client exception

Description of problem:

fluentd multiline exception plugin fails to detect JS client exception

Version-Release number of selected component (if applicable):

fluentd 1.14.6

How reproducible:

Always

Steps to Reproduce:

setup fluentd as collector and configure CLF to detect multiline exception detection.

prepare an appilcation which generates following log lines

Error    
    at bls (<anonymous>:3:9)    
    at <anonymous>:6:4    
    at a_function_name            
    at Object.InjectedScript._evaluateOn (http://<anonymous>/file.js?foo=bar:875:140)    
    at Object.InjectedScript.evaluate (<anonymous>) 

Actual results:

The output gets each exception line as a separate output line

Expected results:

The exception should be combined into one output iine.

Additional info: